This invention relates generally to concrete floating docks and methods for making the same.
Floating dock structures are widely used in marinas and boat harbors as a means for docking and mooring water craft in tidal waters or in other bodies of water subject to changing water level. Typically, floating docks structures are constructed by interconnecting individual or modular float sections. A common arrangement for a floating dock includes a central walkway comprised of float sections connected end-to-end and a series of dock sections projecting perpendicularly from the walkway so as to form a series of boat slips.
Individual float sections are commonly formed by completely or partially encasing a buoyant flotation core, such as polystyrene, in a concrete shell. Over time, traditional concrete float sections have shown their inability to consistently resist the environmental conditions and dynamic forces acting upon them without damage. Traditional concrete floats have also shown their inability to consistently float with adequate freeboard without listing or relative twisting between dock sections. Maximizing freeboard is necessary to ensure that the dock components comprising material susceptible to corrosion and deterioration, such as steel or wood, generally reside above the splash zone of the dock.
Generally, the design of concrete docks is influenced by three factors: (1) the limitations of lightweight aggregate-based concrete and thin concrete sections; (2) the limitations of the strength, flexibility and compatibility of the connections linking dock sections; and (3) the complexity of manufacturing dock sections. The shortcomings of traditional concrete docks are further described with respect to these factors as follows.
Prior art concrete floats utilize thin concrete sections or lightweight aggregate-based concrete to minimize dead weight and thereby increase freeboard. Some float systems utilize both thin sections and lightweight concrete simultaneously.
Lightweight aggregate-based concrete, however, is generally not a preferred material for float sections because of its weak structural properties and porous nature. Thin concrete wall sections are also deficient in that they do not provide sufficient coverage for the embedded steel reinforcing which is susceptible to corrosion when placed in seawater. Corrosion of the steel reinforcing reduces the overall strength of the float and expands and spalls the concrete, further weakening the float and exposing the reinforcing to the elements, which in turn causes additional corrosion.
Floats made from lightweight concrete or with thin wall concrete sections are also vulnerable to freeze-thaw deterioration. This phenomenon occurs when water enters voids in the concrete via superficial cracks and expands upon freezing, thereby enlarging the cracks and spalling the concrete. Enlarged cracks and spalling allow additional water to penetrate the concrete, leading to yet larger cracks and more spalling. In a relatively short time, concrete surfaces can become so damaged by freeze-thaw deterioration that the entire float module must be replaced. The effects of freeze-thaw deterioration are even more destructive in floats made from lightweight concrete because the porous nature of such floats facilitates the absorption of water.
Since it is desirable to cast a concrete float that is both lightweight (to increase freeboard) and durable, it is necessary to increase the thickness of the concrete shell where it is most needed while eliminating other concrete sections to compensate for the added weight. For example, the durability of the protective concrete shell can be improved without increasing weight by eliminating the concrete bottom of the float, which is not required for structural support, so that the thickness of the side walls and deck may be increased. In such a configuration, the exposed portion of the flotation core is typically covered with a protective coating such as polyurethane.
Thicker concrete sections can also be achieved by eliminating redundant end walls in a series of dock sections coupled end-to-end. Conventional floats are typically cast in lengths of 8 to 12 feet. In contrast, monolithic concrete floats can be cast up to 60 feet in length. By increasing the overall length of the individual dock sections, the number of dock sections and thereby the number of end walls adding weight to the dock assembly is reduced. Removing weight in the form of end walls therefore allows for thicker concrete walls and decks.
Concrete float sections are customarily connected to each other by elongated, rigid timber members, or wales, that extend along the upper side edges of the floats. The wales are typically fastened to the floats by tie rods extending transversely through the float and projecting through the wales. The ends of the rods are threaded to receive conventional nuts and washers which are torqued against the wales to compress the wales against the float. Alternatively, U.S. Pat. No. 3,967,569 to Shorter, Jr. (Shorter) discloses float units having horizontally projecting structural flanges that extend along the length of each side wall. The float units are interconnected end-to-end with wooden wales placed on the bottom and top surfaces of the structural flanges and bolted vertically therethrough. Each wale is underlain with a thin steel strap for additional structural strength.
When set in motion by wind or wave action, the heavy mass of traditional concrete floats causes flexing of the wooden wales which in turn, transmits substantial forces to the bolts or tie rods that fasten the wales to the floats. This leads to excessive wear or fatigue of the metal fasteners and as a result, adjacent float modules will eventually come in contact with each other and gradually beat themselves apart. It is difficult to keep the tie rods or bolts sufficiently tight so as to prevent movement of the wales relative to the float units because of the moisture incompatibilities of the wood-steel-concrete connection materials. Once bolt slip or compression of the wood occurs, the holes in the wales enlarge and allow for additional movement of float units, causing failure of the metal fasteners and contact between the concrete surfaces of adjacent floats, which in turn, causes wear and breakage of those surfaces. Further, bolt slip and wood compression causes the fasteners to work loose in the concrete, occasionally pulling out of the float.
Dock attachments, such as cleats and utility stands, are usually bolted directly to the wales because the concrete sections are too thin to accept bolts and are prone to cracking. Attachments subjected to substantial forces, such as mooring cleats, cannot develop their rated capacity because attachment strength is limited by the relatively thin timber members. In addition, to prevent damage to vessels using the dock, connections made with tie rods must not protrude past the mooring face of the float and therefore must be recessed within the side of the wale, which further weakens the connection.
Timber wales are also not capable of resisting the torsional twisting or listing of the assembled float units. The relative twisting between float units is partly due to the inconsistent structural properties of the different materials comprising the float connections and the high water absorption characteristics of the wooden wales. Over time, as the wales continue to creep and the float units become unevenly saturated, the twisting and listing of the float system becomes worse. Twisting and listing of floats is unsightly and is a safety concern in wet or icy conditions.
Flexible joints or hinges are typically used to couple longer, monolithic floats because the flexure forces, concentrated at the joints between float units, would quickly overpower a traditional timber wale-style, rigid connection. One such hinge comprises a steel bolt or cable surrounded by a rubber bushing secured at each end to a float section. This type of connection dissipates bending stresses between float sections but it is also susceptible to wear, fatigue and shock loading.
Quality control of concrete float units is a particular problem associated with the float manufacturing process as manufacturers historically have had problems achieving consistent, balanced flotation. Because a concrete float module is top-heavy and individually unstable, a small variance in concrete wall thickness will lead to undesirable twisting and listing of the assembled float system. Variations in wall thickness are caused by shifting of the flotation core during the casting process. The tendency of the flotation core to shift off-center is often the result of unbalanced hydrostatic pressures created while the concrete is poured and the vibrating mechanism used to settle the concrete. During the manufacturing process, and especially once a float unit has been cast, it is very difficult to determine whether or not the flotation core has shifted. Thus, uneven wall thickness caused by the shifting effect of the core is not apparent until the float has been placed in water.
Moreover, manufacturers often increase the depth of a float unit in order to improve its freeboard. As a float is made deeper, however, it also becomes more unstable so the effects of flotation core shifting become more pronounced.
In addition to flotation problems, shifting of the flotation core reduces the thickness of at least one side wall and/or end wall of the concrete shell, thereby increasing the likelihood of corrosion of the embedded steel and the susceptibility to freeze-thaw damage.
The present invention seeks to overcome the foregoing problems of the prior art by providing an improved concrete floating dock assembly and a method for manufacturing individual float sections. A primary objective of the present invention is to furnish concrete float sections with increased strength and durability that float with adequate freeboard and minimal twisting or listing.
The invention accomplishes this objective with a dock assembly that includes a plurality of interconnected concrete float sections. Each float section comprises a reinforced concrete shell having a deck portion, a pair of opposing longitudinal side walls, a pair of opposing transverse end walls, and an open bottom which together define a flotation container. A flotation core is disposed in the flotation container and has a bottom portion that extends outside thereof.
The concrete shell further includes a pair of opposing, horizontally projecting concrete structural side flanges integral with and extending along the length of each side wall. Similarly, a pair of opposing, horizontally projecting concrete structural end flanges are integral with and extend along the length of each end wall.
By increasing the depth of the flotation core beyond that of the concrete shell, freeboard may be increased without reducing the weight of the float or increasing the depth of the concrete shell. Since the dead weight of the float section is not so limited by freeboard considerations as in prior art float sections, the thickness of the concrete sections comprising the shell may be increased to add strength and durability. Thus, the side flanges and end flanges are of sufficient thickness to permit direct attachment of mooring accessories and dock hinges for interconnecting adjacent float sections. Because attachment strength is not limited by the thickness of the flanges or by the use of wooden wales, each bolt in a dock attachment is capable of developing its full rated capacity. In addition, the use of side flanges and end flanges for the attachment of mooring accessories allows for their easy installation, removal or relocation after the dock assembly has been installed. Further, the increased concrete thickness of the side walls, end walls, and deck portion of the concrete shell reduces its susceptibility to corrosion and freeze-thaw deterioration.
The manufacturing process employed in the invention significantly reduces undesirable twisting and listing of each float section within the dock assembly. To fabricate a float section, a polyurethane coating is applied to the bottom surface of a flotation core which is then placed within a formwork having the shape and size of the concrete shell. Reinforcing bars are then secured in place between the formwork and the flotation core. The flotation core is configured to be self-centering such that each bottom side surface of the flotation core abuts against the inside surface of an adjacent formwork wall. After the flotation core and reinforcing bars are set in place, concrete is poured into the formwork around the sides and top of the flotation core to form the concrete shell. A vibrating mechanism may be employed to facilitate consolidation of the concrete. After the concrete has hardened, the formwork is stripped away.
By having the flotation core in contact with the sides and bottom of the formwork, it remains centered and cannot shift laterally or float upwards regardless of vibration or unbalanced hydrostatic pressures. The flotation core ensures consistent concrete thickness in the side walls, end walls, and deck portion of the concrete shell so that twisting and listing of the float section due to flotation core shifting and uneven wall thickness is eliminated.